In recent years a need has developed for low power, low cost and light weight display devices. EPDs (Electrophoretic Displays) can fulfil this requirement. One use of EPDs is for electronic paper. It is imperative that once an image is displayed, the image can be retained for a long period of time without further voltage being applied. Hence, this fulfils the requirements of low power use, and means an image can be visible until another image is required.
An EPD generally comprises charged electrophoretic particles dispersed between two substrates, each comprising one or more electrodes. The space between the electrodes is filled with a dispersion medium which may be a different colour to the colour of the particles. The space between the electrodes may also be filled with a transparent dispersion medium and two kinds of particles with charge of opposite signs. If a voltage is applied between the electrodes, charged particles move to the electrode of opposite polarity. The particles can cover the observer's side electrode, so that a colour identical to the colour of the particles is displayed when an image is observed from the observer's side. Any image can be observed using a multiplicity of pixels. Available technologies of EPDs include electronic paper, commercially used in electronic books. This application uses black and white or colour. However, the main disadvantage of state of the art EPDs is the lack of a bright full colour system.
The use of different coloured particles in a single pixel has been exemplified in recent patent literature (U.S. Pat. No. 7,304,634, GB 2 438 436, US 2007/0268244), but all of these approaches require the use of complex cell structures and drive schemes. Special coloured particles for EPDs and processes for their preparation are disclosed in US 2007/0297038, US 2008/0013156, U.S. Pat. No. 6,822,782, WO 2007/048721, WO 2008/003619, WO 2008/003604, US 2005/0267263, WO 2006/126120, and J. Nanosci. Nanotechn. 2006, Vol. 6, No. 11, p. 3450-3454. Two particle system comprising inorganic and resin particles are also known (EP 1 491 941). These coloured particles are only achievable by complicated processes and/or they are only suitable for specific applications. Similar coloured particles and their preparation processes are known for analytical techniques (U.S. Pat. Nos. 5,607,864 and 5,716,855) and as toner particles for ink jet printing (U.S. Pat. No. 4,613,559).
In EPD, in order to move particles effectively in a non-polar fluid, and to avoid flocculation of particles, the particles require to be sterically stabilised and charged. Reported methods of preparing polymeric particles suitable for EPD are complicated and have numerous steps. There is a need to simplify the complicated preparation of polymeric particles suitable for EPD a simple preparation of charged coloured particles which can be easily dispersed in non-polar media, show electrophoretic mobility and which do not leach colour in a dispersant. Therefore, the object of this invention is to provide electro-optically active media for colour electrophoretic displays and specifically engineered coloured particles for use in such media.
This object is solved by polymer particles for use in electrophoretic devices comprising at least one A-B diblock copolymer comprising a hydrophobic polymer block A and a hydrophilic polymer block B containing a charge or being chargeable, and monomer units of at least one monomer, of at least one polymerisable dye, optionally of at least one charged co-monomer, and optionally of at least one cross-linking co-monomer, by a process for the preparation of polymer particles for use in electrophoretic devices, comprising a) the reaction of at least one monomer, at least one A-B diblock copolymer, at least one initiator, optionally at least one polymerisable dye, optionally at least one charged co-monomer, and optionally at least one cross-linking co-monomer, b) optionally colouring the polymer particles by incorporation of at least one dye and/or at least one pre-polymerised dye and/or at least one polymerisable dye, and optionally c) washing the polymer particles, by these particles per se, by the use of these particles for the preparation of an electrophoretic device, by electrophoretic displays comprising such particles, and new polymerisable dyes.